The present invention relates to micromechanics and microsensors. More particularly it relates to a micro electromechanical optical inertial sensing device.
Micromechanics is a scientific and technological field dealing with the research and development of microsystems and microelements that incorporate sensing, controlling, and actuating, using microelectronics technology. The common basic material for such devices is silicon, and the dimensions of devices incorporating VLSI approach microelectronics are usually in the range of 10 xcexcm to 1000 xcexcm. These devices incorporate control and logic elements, and are designed to operate under various performance regimes, these being mechanical, electrical, optical, biological, thermal, fluidal, chemical or nuclear. Micromechanical devices are meant to be small, cheap to produce and ideal for mass production. These properties render micromechanical devices advantages in terms of performance, reliability and cost saving of the grater system mostly, where the micro device dictates its overall minimum size.
Production technologies of micromechanical devices involve several known methods of micro-processing: photolithography, wet etching, dry etching, evaporation, sputtering, deposition, oxidation, etc. Commonly used production technologies are: surface micromachiningxe2x80x94microprocessing of thin film layers, as opposed to bulk micromachiningxe2x80x94microprocessing of bulk materials; hybrid micromachiningxe2x80x94the mechanical device is attached to a microelectronic chip, as opposed to monolithic micromachiningxe2x80x94the mechanical devices and the electronics are incorporated on the same chip; LIGAxe2x80x94the third dimension is shaped using X-ray lithography and constrained electroplating.
Inertial sensing devices are used in various applications like avionics, missile guidance systems, vehicle safety systems (airbags), spacecraft guidance and navigation systems, etc. In many of these applications, overall size and cost depend critically on the size of the navigation system. Effort is thus being put into the prospects of providing micromechanical inertial sensing devices, in order to furnish cheap, mass produced, accurate, very small such devices.
Micromachined devices that are based on the dynamic response of their particular structure include accelerometers, pace meters, pressure sensors, moisture gauges, and others. In such devices the typical dynamic parametersxe2x80x94such as the resonance frequency, Q factor, amplification, and dampingxe2x80x94present means for measuring the independent variable, it being acceleration, partial pressure and others. Although the basic motion laws apply in three-dimensional structures, in the micron and submicron scales, non-linear effects, which are often negligible in the macro-environment, become dominant. Among others, these non-linear effects include damping mechanisms unique for the specific micromechanical geometries, and external loads, which occur, for example, as a result of an electrostatic excitation and the parasitic effect of capacitive sensing.
Micromechanical proof-mass devices can be designed to move in any of the six degrees of freedom. These movements result both from internal excitation and/or external physical loads. Common methods of excitations are electrostaticxe2x80x94vertical, horizontal (as in comb-drive) or tilt; piezoelectric; optical; and others. The movements caused by the physical measure, which can be pressure, angular rate, acceleration, must be sensed by the device in order to estimate it.
There are several common sensing methods used in micromachined devices. One method is the capacitive sensing, where the movement of the microstructure changes the relative position of one capacitor plate relatively to the other, thus changing the capacitance and measuring these changes. A slightly different sensing approach is implemented using a comb-like structurexe2x80x94a structure of comb-like fins joined by a firm base, which are incorporated with an opposite comb-like structure attached to a static frame. The movement of the microstructure changes the overlapping area of the corresponding fins, resulting in a change in the total capacitance.
Another method is the piezoelectric sensing, where a change in the electric field inside crystallic matter is induced by a mechanical strain, and is recorded. Yet another sensing method is the piezoresistive sensing, where the change in the resistance of the piezoresistive layer is recorded, known to be dependent on the mechanical strain.
An optical sensing method can be used in conjunction with a membrane. A laser light beam is directed at the membrane and its change in deflection, or its reflected intensity is measured, depending on the nature of the surface curvature imposed by the dynamic motion of the membrane.
There are two main factors that limit the MDS (Minimum Detectable Signal). One factor that exists in all sensing systems is noise. There are several noise mechanisms in micromechanical devices such as: mechanical noise, electronics noise, light noise, etc. The noise sources can be either xe2x80x98whitexe2x80x99 (frequency independent) or xe2x80x981/fxe2x80x99 (xe2x80x9cfxe2x80x9d for frequency). Another factor that exists in some of the sensing methods is cross-talk between the sensing mechanism and the dynamics of the microstructure, e.g. in capacitive sensing.
It is the object of the present invention to provide a micromachined optical sensing device for the detection of rate and acceleration, using optical means that are independent of the dynamics of the micromechanical device, thus simplifying the analysis and improving sensitivity.
The micro electromechanical optical inertial sensing device of the present invention comprises
a CMOS chip comprising at least one integrated photodiode and analog electronics;
an elastically suspended proof mass, hybridically attached to a CMOS chip, over the chip,
a specified light source, directing a light beam at the suspended proof mass, on the side facing away from the CMOS chip, thus casting a partial shadow over said photodiode, when the suspended proof mass is at rest;
a signal processing circuit for the on chip processing of the photodiode output signal; and
mechanical and electrical connecting means, to join the proof mass component with the CMOS chip component;
wherein said photodiode is positioned substantially along the imaginary coaxis of the assumed measured movement and said proof mass, said photodiode being electrically connected to the processing means, wherein when the device is subjected to an inertial movement, the suspended proof mass swings, thus causing the partial shadow casted on the photodiode to shift, and modulate the light beam illumination on the CMOS integrated photodiode, generating subsequent output current signal transmitted to and processed by analog electronics processing means, to produce meaningful measurement results, or to generate electronic signals to other systems.